1. Field of the Invention
The present invention relates to an optical disc medium mountable on an erasable and rewritable optical disc device, particularly a structure of an erasable and rewritable optical disc medium which can be used in place of a compact disk (CD) medium usable for reproduction.
2. Description of the Related Art
Remarkable progress has been made in the developments of optical disc devices which reproduce, record and erase information by using laser beams, and optical disc media to be used for such devices, and development of this technique has made them promising media for memory devices to be used in high performance computers. Also, as laser beam utilizing recording devices for domestic use, CD devices are now very popular, and the use of a CD-ROM as a ROM medium in personal computers is also widely spread.
In view of the state of the art, if an erasable or additionally writable optical disc medium exchangable with a standard CD medium existed, it could be used for many purposes, such as the preparation of voice information distributed in small amounts, copying voice information, backing-up a CD-ROM medium, using the CD-ROM medium as a RAM, etc., but to be utilized as described above, adequate medium structure must be adopted.
In the prior art optical discs (CD, CD-ROM, LD) used for reproduction only, information is recorded by forming irregularities on a plastic substrate, and the structure is such that information stored by forming these irregularities is reproduced by irradiating a laser beam from the substrate side. In this kind of reproduction, however, the user cannot record or write new information onto the optical disc.
Further, the optical constant of the phase transition type optical disc medium is reversibly or irreversibly changed by an irradiation of a laser beam, but in practice, the reflectance in the writing and erasing state is 40% or less. Accordingly, it is difficult to use the same in place of a CD medium for which a reflectance of 70% or more is required.
Accordingly, a CD exchangeable optical disc medium utilizing an erasable or additionally writable phase transition type has not been provided, and a utilization method such as mentioned above is not available, although, U.S. Pat. No. 4216501 discloses a pit formation type information record and Japanese Unexamined Patent Publication No. 57-111839 discloses a reversible type information record.